1. Field of the Invention
The present invention relates to a power supply device that synchronously rectifies three-phase AC voltages and thereby charges up a battery.
2. Description of the Related Art
A conventional power supply device of this kind disclosed in, for example, Japanese Laid Open Patent No. H11-225446 (hereinafter referred to as Patent Document 1) is shown in FIG. 9, in which when the voltage of a battery 7 is low (the battery 7 is not fully charged up): more specifically, when a voltage detection means 6 detects that the battery voltage is lower than a predetermined voltage, thereby outputting no signal (referred to as a charge-permit state here), that is, when the battery is in this charge-permit state, a switch control means 5 outputs signals to semiconductor switches Q1 to Q3 so as to turn on each switch to become short-circuit during a period of a negative voltage detection means 3, which detects three-phase AC voltages generated by a generator 1 being negative, outputting signals.
Next, during a period of the negative voltage detection means 3, which detects the three-phase AC voltages generated by the generator 1 being negative, outputting no signal, the switch control means 5 outputs signals to semiconductor switches Q1 to Q3 so as to turn off each switch to become nonconductive. Therefore, the three-phase AC voltages from the generator 1 are synchronously rectified through diodes D1 to D3 and the semiconductor switches Q1 to Q3, and then the battery 7 is charged up with the rectified voltage.
Meanwhile, when the battery is fully charged up and its voltage is high enough: more specifically, when the voltage detection means 6 detects that the battery 7 voltage is higher than the predetermined voltage, outputting a signal (referred to as a charge-stop state here), the switch control means 5, in this charge-stop state, inputs its output signals to the respective semiconductor switches Q1 to Q3 at a time during a period of no current flowing through each of the three phases of the generator 1 (for example, during a period of the negative voltage detection means 3, which detects the three-phase AC voltages generated by the generator 1 being negative, outputting signals), whereby the semiconductor switches Q1 to Q3 are held in an ON state and made conductive until the charge-permit state begins. Therefore, in the charge-stop state, regardless of the three-phase AC voltages output from the generator 1 being positive or negative, the semiconductor switches Q1 to Q3 become conductive, so that the battery is put into a non-charging state. For a while afterward, the non-charging state is continued. When the battery voltage becomes lower than the predetermined voltage, the same operations as those in the charge-permit state are repeated.    Patent Document 1: Japanese Laid Open Patent No. H11-225446
In FIG. 10, in the charge-stop state where a battery voltage 13 is higher than a predetermined voltage 12, the semiconductor switches Q1 to Q3 are made conductive at a time during the period of no current flowing through each of the three phases of the generator 1 (for example, during a period of the negative voltage detection means 3, which detects the three-phase AC voltages generated by the generator 1 being negative, outputting signals), and continue to stay in the conductive state, thereby short-circuiting also the following positive voltages; therefore, the voltage of a phase whose voltage has been short-circuited earlier than the other phases becomes higher than those of the other phases, which causes, as shown in FIG. 10, voltage imbalance between the three-phase AC voltages 11U, 11V and 11W output from the generator 1.
Then, by making the semiconductor switches Q1 to Q3 conductive, the battery voltage 13 is lowered, putting the battery into the charge-permit state, whereby the switch control means 5, during a period of each of the AC voltages being negative, turns on their respective semiconductor switches Q1 to Q3 to become conductive so as to resume synchronous rectification, and then charging up of the battery 7 starts. However, the voltage imbalance between the three phases that has arisen during the charge-stop state continues for several periods immediately after the state changing from the charge-stop to charge-permit state; therefore, in a phase that has been firstly short-circuited, a negative voltage sometimes cannot be detected for these several periods. If the charging up starts without detecting the negative voltage, a phase in which the negative voltage cannot be detected cannot be short-circuited, and furthermore, the voltage at the phase in which the negative voltage cannot be detected stays high making it unable to detect a negative voltage because the charging up of the battery has already started. The switch corresponding to the phase in which the negative voltage cannot be detected is continuously held conductive, thereby causing problems in that the semiconductor switch is broken down precluding the synchronous rectification, so that the battery 7 cannot be charged up.